The Trump-Russia probe deepens as special counsel asks White House to hand over documents

Although not a formal subpoena, the request for documents is the first known instance of Robert Mueller’s team asking the White House to hand over records

- In this June 21, 2017 file photo, Special counsel Robert Mueller after a closed-door meeting on June 21, 2017, with members of the Senate Judiciary Committee about Russian meddling in the election aShow more

Investigators working for US special counsel Robert Mueller, who is leading the inquiry into alleged collusion between Donald Trump's 2016 presidential campaign and Russia, have asked the White House for documents related to his former national security adviser Michael Flynn.

The New York Times reported that the document request - although not a formal subpoena -  is the first known instance of Mr. Mueller’s team asking the White House to hand over records. Citing unnamed sources close to the investigation, the Times also said the investigators had questioned witnesses about whether Mr Flynn was secretly paid by the Turkish government during the final months of the 2016 campaign.

The White House and Mr Flynn's lawyer, Robert Kelner, declined to comment but Ty Cobb, special counsel to Mr Trump, said, “We’ve said before we’re collaborating with the special counsel on an ongoing basis.”

President Trump has denied any collusion between his campaign and Russia and labelled the investigation a witch hunt.

On Saturday, the former Russian ambassador to the United States strongly denied the accusation of meddling in the American presidential race. Sergei Kislyak, who has just returned from Washington, said  he was merely doing his job as a diplomat when he met with members of President Trump's team. He had also had meetings with representatives of Hillary Clinton's campaign, but did not name names.

Mr Kislyak described the accusations against him as absurd and "shameful" for the US. The official acknowledgement that his phone conversations were bugged was "unhealthy," he added.

"Any diplomat, Russian or not, works to better understand the policy of a country he's posted to, figure out what the new administration's course is and understand where cooperation is possible," the former ambassador said.

Asked about his contacts with Michael Flynn, who served briefly as Trump's national security advise, Mr  Kislyak said that they did not  discuss any secrets.

"We talked about very basic things," he said. "There are a few subjects important for Russia-US cooperation, primarily terrorism, and it was one of the subjects we talked about. Our conversations were legitimate, calm and absolutely transparent."

He insisted they had not talked about sanctions because he was under orders from Moscow not to discuss the subject. "I had instructions not to discuss sanctions," he said. "We haven't been involved in any discussions or bargaining over sanctions, because we believe that they have been introduced unlawfully."

The president sacked Mr  Flynn in February after it became clear that he had misrepresented the nature of phone conversations he had with the Russian ambassador last  December. in June it was reported that federal prosecutors in the state of Virginia were investigating a deal between Mr Flynn and Turkish businessman Ekim Alptekin as part of a grand jury criminal probe.

Meanwhile, a White House adviser raised the possibility of lie detector tests for the small number of people in the West Wing (the president's work quarters) and elsewhere with access to transcripts of Mr Trump's phone calls. Kellyanne Conway suggested the measure after transcripts were published of  Mr Trump's telephone conversations with the leaders of  Mexico and Australia.

Appearing on the television show,  "Fox & Friends" , Ms Conway said, "It's easier to figure out who's leaking than the leakers may realize."

Attorney General Jeff Sessions pledged on Friday to rein in government leaks that he said undermine American security.